Optical communication technology has moved from simple amplitude modulation (AM) to more advanced modulation techniques using both amplitude and phase. With increasing demand for higher throughput, optical communication systems have adopted these more advanced modulation formats which require increasing spectral efficiency of the system. One of these formats is differential QPSK (DQPSK), in which information bits are coded as phase transient between adjacent symbols. DQPSK has a high tolerance for phase noise.
One simple way to decode a DQPSK signal is by using an analog DQPSK decoder, sometimes referred to as an optical delay interferometer. With reference to FIG. 1, there is illustrated such a prior art decoder or interferometer in which the optical signal is delayed and added before detected by an intensity detector. However, the drawback to this simplicity is that only amplitude is detected and all of the phase information of the received signal is lost. With increasing baud rates, the signal is more and more sensitive to link impairment, such as dispersion and Polarization Mode Dispersion (PMD), which introduces amplitude and phase distortion to the optical signal. With only signal amplitude being detected, there is no effective method to compensate for these impairments and increase performance.
To recover both amplitude and phase of the received optical signal, coherent detection techniques have been widely adopted in new generation optical communication systems. Turning to FIG. 2, there is illustrated the basic structure of a coherent detection system or receiver. The outputs of this receiver are two electrical signals corresponding to the in-phase (I) and the quadrature (Q) of received optical signal Es.
In the coherent detection technique, an important aspect is carrier phase recovery or estimation (CPR) which recovers and compensates for phase noise in the received optical signal, thus enabling recovery of the information data. FIG. 3 illustrates an example structure of a prior art CPR. For DQPSK signals, a digital differential decoding module is usually implemented after the CPR. There are drawbacks to using coherent detection with CPR. Implementation of the CPR can be quite complex, and a CPR's bandwidth is limited by hardware feasibility. As a result, CPR is limited and cannot handle high phase noise with wide bandwidth.
In an optical communication system, there may be present some specific link conditions which could lead to high phase noise due to fiber nonlinearity. In these applications, the conventional CPR (feed forward or backward) does not provide a large enough bandwidth to compensate for the phase distortion. The inventors have determined that one possible way to effectively overcome these issues is to utilize a digital delay interferometer (similar to FIG. 1) but utilize a digitally recovered signal to replace the analog optical signal. Compared to the analog optical interferometer mentioned above, there would be several advantages. First, existing DSP techniques can be used to compensate for link impairments. Second, CPR can be eliminated thus dramatically simplifying implementation and reducing cost. Lastly, this method can tolerate high phase noise.
However, in such a proposed system, the recovered electrical signals may incur a constant phase ramp because of frequency offset between the laser in the transmitter and the laser utilized as the local oscillator (e.g., LO in FIG. 2).
In the reception of optical signals by an optical signal receiver, unwanted laser phase noise and frequency offset resulting from application of a local oscillator laser in the receiver (that is different from the laser in the transmitter) are injected into the received optical signals. These must be removed (or substantially reduced) prior to demodulation to enable successful recovery of the information data. Because of this, optical transmitters and receivers have commonly utilized oscillator lasers with low phase noise. These low phase noise oscillator lasers are expensive. Therefore, there is needed a method and system that improves coherent detection in optical communications systems that implement advanced modulation formats while also reducing system cost by enabling the use of less expensive local oscillator lasers.